A voltage-controlled oscillator is an important component module of an analog circuit and a digital circuit. The voltage-controlled oscillator may be implemented in many different manners. Compared with a conventional LC oscillator, a ring oscillator occupies a smaller chip area and has a larger tuning range. If the ring oscillator includes two-stage delays, the ring oscillator can work at a high frequency, and provide quadrature outputs.
In a modern CMOS technology, as technical features, a size and a supply voltage need to be reduced in proportion to maintain stability of a device. The ring oscillator hardly works at a 0.5 V supply voltage due to a high-threshold voltage of a transistor. Connecting a substrate of a MOS transistor to a forward bias is an effective method for reducing the threshold voltage of the transistor. In a design, a substrate forward bias technology is applied to a delay unit having a local positive feedback.
Therefore, a great progress is urgently needed in the prior art.